The United States and Afghanistan are talking about whether an American residual force should remain after combat operations end in 2014.

The "zero option" - no U.S. troops after 2014 - "has always been available," Carney said, but the final decision will be based on "political objectives" that include the stability and security of Afghanistan.

The New York Times reported that Obama is examining the zero option because of frustration with Afghan President Hamid Karzai.

Karzai has criticized the United States for opening talks with the opposition Taliban.

Carney said all diplomatic relationships have their ups and downs, and final decisions will be based on what is best for Americans and Afghans.

However many troops stay in the country, Carney said, "we will continue to be committed to Afghanistan beyond 2014."